1. Field of the Invention
The present invention relates to a silicon wafer and its manufacturing method. More particularly, the present invention relates to a silicon wafer having an amorphous silicon layer, formed as a gettering layer, on the main surface (hereinafter referred to as a "reverse surface") opposite to the main surface (hereinafter referred to as a "front surface") on which semiconductor devices will be fabricated, as well as to a method of manufacturing the silicon wafer.
2. Description of the Related Art
For various reasons, impurities such as heavy metals are generated during fabrication of a semiconductor device from a silicon wafer, and the thus-generated impurities contaminate the vicinity of the surface of a silicon wafer which will become an active area in the semiconductor device. If the vicinity of the surface of the silicon wafer which will become an active area is contaminated by impurities, the characteristics of the device are degraded, which in turn reduces the proportion of non-defective devices. To prevent such a problem, a technique of intentionally collecting impurities outside of the active area; namely, a gettering technique, is commonly used. A representative method of the gettering technique is the forming of a polycrystalline silicon layer as a gettering layer on the reverse surface of a silicon wafer.
According to this conventional method, a polycrystalline silicon layer is usually formed by growing polycrystalline silicon on the reverse surface of the silicon using a LPCVD (Low Pressure Chemical Vapor Deposition) method.
The silicon wafer having a polycrystalline layer formed on the reverse surface thereof undergoes stress. Compared with a silicon wafer without a polycrystalline silicon layer, the silicon wafer having a polycrystalline layer becomes warped to a greater extent. If polycrystalline silicon is grown on the reverse surfaces of silicon wafers of different diameters under the same conditions, the warpage increases with the diameter of the silicon wafer. Associated with a recent tendency of a semiconductor device to be highly integrated with higher accuracy, the diameter of the silicon wafer is steadily increasing. Because of this, it is to be desired that the warpage of the silicon wafer be reduced to as small an extent as possible.
In order to reduce the warpage of the silicon wafer, a polycrystalline silicon layer is formed on a silicon wafer under conditions such that the stress acting on the silicon wafer decreases. For example, the polycrystalline silicon layer is formed on the silicon wafer at a high temperature such that the grain size of polycrystalline silicon becomes greater, or the polycrystalline silicon layer is formed so as to become thinner. In these cases, however, the gettering capability of the polycrystalline silicon layer is decreased, as shown in FIG. 1.
As a polycrystalline silicon layer is subjected to heat treatment during fabrication of semiconductor devices from a silicon wafer, the polycrystalline silicon layer is further crystallized into monocrystal, thereby resulting in a reduction in the gettering capability of the polycrystalline silicon layer.
There has been proposed a silicon wafer whose gettering capability is enhanced by formation of an amorphous layer instead of the polycrystalline silicon layer (see Japanese Patent Application Laid-Open (kokai) No. 4-2133). As shown in FIG. 5, in addition to the advantage of improved gettering capability, the amorphous silicon layer has the advantage of being resistant to crystallization into monocrystal relative to the polycrystalline silicon layer, even when it is subjected to heat treatment during the fabrication of semiconductor devices, thus providing the advantage of longer continuance of the gettering capability.
As illustrated in FIG. 2, if an amorphous silicon layer is formed (designated by (b) in FIG. 2) by growing amorphous silicon on the reverse surface of the silicon wafer using the low pressure chemical vapor deposition method in the same way as for polycrystalline silicon, the stress acting on the silicon wafer becomes larger than the stress resulting from formation of a polycrystalline silicon layer (designated by (a) in FIG. 2). As a result, the warpage of the silicon wafer becomes still greater. As illustrated in FIG. 1, as is the case of formation of a polycrystalline silicon layer, even if an amorphous silicon layer is formed using the low pressure chemical vapor deposition method, the stress acting on the silicon wafer increases so long as the gettering capability of the amorphous silicon layer increases.
To grow amorphous silicon on the reverse surface of the silicon wafer, it is necessary to reduce growth temperature. As a result of this, as illustrated in FIG. 3, a growth rate of the amorphous silicon (designated by (b) in FIG. 3) becomes extremely low as compared with that of the polycrystalline silicon (designated by (a) in FIG. 3), thereby resulting in a decrease in the productivity of a silicon wafer.